Inorganic and organic porous particles have been prepared and used for decades for many different purposes. For example, porous particles have been described for use in chromatographic columns, ion exchange and adsorption resins, drug delivery devices, cosmetic formulations, papers, and paints. The methods for generating pores in organic and inorganic particles are well known in the field of polymer science. However, each type of porous particle often requires unique methods for its manufacture. Some methods of manufacture produce large particles without any control of the pore size while other manufacturing methods control the pore size without controlling the overall particle size According the International Union of Pure and Applied Chemistry (IUPAC, www.iupac.org), micropores, mesopores, and macropores refer to pores with diameters of below 2 nm, from 2 to 50 nm, and above 50 nm, respectively.
Many applications use inorganic materials containing both mesopores and macropores. Inorganic porous materials generally exhibit advantages of higher mechanical strength, higher thermal stability, and higher chemical durability than those derived from organic polymers. These features meet the demands of a high temperature and high pressure operation of separation or reaction processes favored in a large scale production. The sol gel process is commonly used to prepare inorganic porous materials because of its ability to form inorganic networks from silicon or other metal alkoxides with desirable hardness, optical transparency, chemical durability, tailored porosity, and thermal resistance using a room temperature process. Products made from sol-gel technology include optics, protective and porous films, optical coatings, window insulators, dielectric and electronic coatings, high temperature superconductors, reinforcement fibers, fillers, and catalysts. Many uses exist for inorganic materials with mesopores and macropores prepared from the sol-gel process. Examples include inorganic films and membranes for microfiltration and ultrafiltration of beverage and drinking water purification and wastewater treatment, porous structures as catalysts and enzyme supports, and porous monolith chromatography columns as separating media for liquid and gas mixtures. Monolithic macroporous silica with appropriate mesopores has proved to be an efficient separation medium in liquid chromatography. A monolith is a continuous piece of highly porous material usually created by in situ polymerization of a monomeric solution and characterized by a defined pore structure consisting of large flow-through macropores for high permeability and small diffusion mesopores for desired surface area providing high loadability. While the macroporous structure is formed through concurrent phase separation and gelation in the course of hydrolysis and polycondensation of alkoxysilanes in the presence of organic additives, the mesopore structure is tailored by post gelation treatments such as solvent-exchange and accelerating the rate of condensation.
While inorganic monoliths, films, and coatings with a double-pore structure prepared from the sol-gel process are common, inorganic particles with this double-pore structure are less common, because of the difficulty in maintaining the internal macropore structure due to collapse of the pores from the resulting high stresses occurring within the particle during drying. Useful porous inorganic microspheres have been prepared using sol-gel reactions and used as “microreactors” to deliver controlled release of cosmetics, vitamins, or reactive chemicals. Such inorganic porous particles have been prepared using oil-in-water-in-oil emulsions. See for example Lee et al., J. Coll. Interface Sci. 240, 83-89, 2001, in which retinol is entrapped within the macropores of the inorganic porous particles. These particles essentially have one set of mesopores pores, derived from the porous matrix. The larger macropores formed from the first emulsion are completely filled with material preventing collapse during drying. Ettiyappan, P. et al., Colloids and Surfaces A: Physicochem. Eng. Aspects, August 2010 disclose inorganic particles formed using a single water-in-oil emulsion process that inverts to form an oil-in-water-in oil sol-gel multiple emulsion where the first inner oil-in-water emulsion is destabilized to form hollow particles.
It is also known to include marker materials in organic porous particles so that the particles can be detected for a specific purpose. For example, U.S. Patent Applications 2008/0176157 (Nair et al.) and 2010/0021838 (Putnam et al.) and U.S. Pat. No. 7,754,409 (Nair et al.) describe organic porous particles and a method for their manufacture, which organic porous particles are designed to be toner particles for use in electrophotography. Such organic porous particles can contain a colorant such as carbon black or another pigment to provide desired black-and-white or color electrophotographic images. The organic porous particles can be prepared using a multiple emulsion process in combination with a suspension process (such as “evaporative limited coalescence”, ELC) in a reproducible manner and with a narrow particle size distribution.
While various organic porous particles have been prepared for such uses, it is very difficult to prepare inorganic porous particles with two sets of pores with different sizes using an oil-in-water-in-oil sol-gel multiple emulsion processes, due to the high capillary pressures during drying of the sol-gel matrix that result in shrinkage and collapse of pores. In some cases the dry particles have little or no pores greater than 100 nm in diameter.
There exists a need for a method of preparing inorganic porous particles with two sets of pore sizes in which the pores are essentially empty to act as a scaffold for addition or adsorption of useful materials. Thus, there is a need to stabilize the internal macropores in inorganic porous particles prepared by an oil-in-water-in-oil sol-gel multiple emulsion process to maintain the internal integrity of the inner emulsion and macropores during particle formation and drying.